1st Please stop calling them rims. they are wheels, not rims.
2nd Fuel econ goes hand in hand with performance. better spray patterns from upgraded nozzles, better boost maps from better more modern turbos, cooler air charge from bigger FMIC, better airflow on a better intake manifold, so on and so forth. if you do not increase the fuel use, but increase how it burns, you get more power and more MPG. Whenever you do a power mod, you end up getting a jump in MPG as long as your not going full at it. Of Course a tune is the best way to maximize this with supporting mods, but i was able to get 62mpg on average going cross country at 75mph cruise control with my 764's gt2052, and pd150 intake and a bunch of supporting mods on 205/40ZR-17 SUMITOMO HTR Z II 17x8 Axis OG Europa SDC with NO TUNE what so ever (other than the stock one) when i put the stage 6 tune in, the best i could do was about 40, but that was because race car and i didnt care, but i still was at like 230 HP at the wheels so.......................
Full list of mods and a video
still got a normal in town mix/highway mix of about 56 if i drove like a grandma, 8mpg track, about 48 the way i drove it lol